Gas turbine engines include one or more compressors for pressurizing, a working medium fluid, typically ambient air, that flows through a longitudinally extending compressor flowpath. Under some operating conditions, it is desirable to temporarily moderate the pressure at the discharge end of the compressor to prevent or recover from compressor stall or other aerodynamic instabilities. Pressure moderation is usually effected by opening a compressor bleed valve that diverts a portion of the pressurized fluid from the discharge end of the compressor flowpath into a lower pressure region.
An exemplary compressor bleed valve is described in U.S. Pat. No. 4,827,713, which is assigned to the assignee of the present application, and whose contents are incorporated herein by reference. The disclosed valve includes a stationary orifice ring hating a pair of resilient seal members adhesively bonded into respective channels on longitudinally facing surfaces of the ring. A series of circumferentially distributed passages extends through the orifice ring to join the compressor flowpath to a surrounding annular chamber. The compressor valve also includes a moveable valve ring with a cylindrical sleeve and a pair of seal seats radially aligned with the orifice ring seal members. A set of pins extends radially from the valve ring, and each pin includes a roller that engages a carved slot on the orifice ring. A bellcrank for operating the valve ring is mounted on a bellcrank support bracket by a bellcrank pivot. Input and output arras of the bellcrank are connected respectively to an actuator (not illustrated in the reference) and to the valve ring.
In operation, the actuator rotates the bellcrank about the bellcrank pivot so that the bellcrank, in turn, drives the valve ring in a spiral motion, positioning the sleeve to cover or uncover the passages. The rollers help guide the valve ring in its spiral path. As the valve ring approaches its fully closed position, the seal seats contact the seal members, compressing them in the longitudinal direction to effect a fluid tight seal. However because the valve ring moves with a spiral motion, the seats also exert a circumferentially directed shearing force on the seal members. Over time, repeated application of the shearing force can erode the seal members, or can peel or tear the members out of their channels, compromising the integrity of the seal and enabling working medium fluid to leak through the valve, even when the valve is in its fully closed position.
In an alternative configuration of the valve, not shown in the reference, the seal members are adhesively mounted on the valve ring. In yet another alternative configuration, also not shown, one seal member is mounted on the valve ring and one on the orifice ring. However these alternative configurations in no way ameliorate the potential for damaging the seal members since valve closure inevitably produces relative circumferential shearing motion between the seal members and the seal seats.
Damage to the compressor valve seal members, as described above, is undesirable for a number of reasons. The fluid leakage degrades the efficiency of the compressor, increasing engine fuel consumption and operating expense. Collateral damage can occur to engine components located in either the annular chamber surrounding the compressor flowpath, or in a secondary flowpath connected to the annular chamber, since those components are not necessarily tolerant of extended exposure to the elevated temperature of the working medium fluid (sometimes in excess of 400 degrees F.). If the liquid leakage becomes severe enough to warrant replacement of the seal members, further difficulties are encountered because the compressor valve is not readily accessible in a fully assembled engine. Once the engine is partially disassembled to gain access to the valve, the deteriorated seal members are troublesome to remove since the repair technician is obliged to disbond the seal members from their channels, and laboriously cleanse the channels of any residual adhesive. The subsequent reinstallation and bonding of new seal members can be similarly tedious and labor intensive.
Use of the above described valve arrangements in the latest generation of turbine engines will only exacerbate the shortcomings of the sealing arrangement. The compressors in these newer engines pressurize the working medium fluid to pressures and temperatures higher than those typically seen in older generation engines. The higher temperatures weaken the adhesive bond holding the seal members in place and therefore increase the likelihood that the seal members will be peeled out of the channels. In addition, the valve actuation system of newer generation engines applies a greater longitudinal compressive force to the valve ring and seal members, resulting in a correspondingly greater shearing force acting on the seal members during valve closure.
One further disadvantage of the prior art valve occurs because the bellcrank exerts an actuation force on the valve ring at only a single circumferential location. As a result, the valve ring deflects slightly during valve closure so that a relatively tight seal is formed at locations circumferentially close to the bellcrank, and a looser, less effecitive seal is formed at locations circumferentially displaced from the bellcrank. Although this problem could be addressed by applying actuation forces at multiple circumferential locations, the accompanying additional hardware, and the possible need to synchronize multiple actuators, makes this solution prohibitively complex and undesirable. A simpler solution is to circumferentially bias the longitudinal position of the valve ring so that when the valve ring is not in its fully open position, the longitudinal spacing between each seal member and its seal seat varies circumferentially. According to the prior art, the variation in longitudinal spacing is symmetric relative to a diametral line through the bellcrank attachment location. That is, the longitudinal spacing is equal at locations that are equiangularly displaced in opposite directions (clockwise and counter clockwise) from the bellcrank. This solution has also proved to be unsatisfactory since it fails to recognize that valve ring deflections arising from bellcrank actuation forces are not distributed symmetrically about the diametral line.
In view of the above described shortcomings an effective, highly durable, easily maintainable valve seal assembly is sought.